Flash and other types of electronic memory devices are constructed of memory cells operative to individually store and provide access to binary information or data. The memory cells are commonly organized into multiple cell units such as bytes which comprise eight cells, and words which may include sixteen or more such cells, usually configured in multiples of eight. Storage of data in such memory device architectures is performed by writing to a particular set of memory cells, sometimes referred to as programming the cells. Retrieval of data from the cells is accomplished in a read operation. In addition to programming and read operations, groups of cells in a memory device may be erased, wherein each cell in the group is programmed to a known state.
The individual cells are organized into individually addressable units or groups such as bytes or words, which are accessed for read, program, or erase operations through address decoding circuitry using wordlines and bitlines. Conventional flash memories are constructed in a cell structure wherein one or more bits of information or data are stored in each flash memory cell. In typical memory architectures, each cell typically includes a MOS transistor structure having a source, a drain, and a channel in a substrate or P-well, as well as a stacked gate structure overlying the channel. The stacked gate may further include a thin gate dielectric layer (sometimes referred to as a tunnel oxide) formed on the surface of the P-well.
The stacked gate also includes a polysilicon floating gate overlying the tunnel oxide and an interpoly dielectric layer overlying the floating gate. The interpoly dielectric layer is often a multilayer insulator such as an oxide-nitride-oxide (ONO) layer having two oxide layers sandwiching a nitride layer. Lastly, a polysilicon control gate overlies the interpoly dielectric layer.
Other types of memory devices include ones comprising silicon or polysilicon above and below an ONO layer, these silicon-oxide-nitride-oxide-silicon devices are sometimes referred to as SONOS memory devices.
SONOS is operated in various ways. In one typical example, (Fowler-Nordheim) FN tunneling is used to program and erase, wherein charge is injected into the nitride in programming and removed from the nitride in erase. The stored charge in the nitride is uniform in this case. By changing the amount of charge stored in the nitride, multiple data may be recorded in one cell.
In another typical case, hot electrons in the channel are used to program, where charge is stored locally in the nitride at the drain side. Switching the drain and source allows storage of two bits in one cell, thus implementing multi-bit or dual-bit memory cells. In order to erase, hot holes generated at the drain or/and source side is injected in the nitride to neutralize the charge. FN tunneling may be used to erase instead of the hot hole injection.
Appropriate programming and erase schemes are chosen in consideration of the application or the purpose, but structurally the same memory cells may be used.
Core cells in flash memory devices may be interconnected in a variety of different configurations. For instance, memory cells may be configured in a NAND type memory configuration, series connected source to drain along columns of conductive bitlines, with control gates connected for selection along rows of wordlines. FN tunneling is typically used to program and erase in a NAND type memory.
Conventionally, one end of each bitline of the NAND array is connected to a common source line. In particular, select drain gate transistors are used to couple associated cells of a bitline to a bitline contact, while each of the bitlines are coupled via a select source gate transistor to a common source line. Typically, segments of the common source line for each bitline are then locally interconnected together by a conductive local interconnect structure and to a VSS supply contact. In operation, individual flash cells and the individual data bits thereof, are addressed via the respective bitlines connected to first and second source/drain regions thereof and a wordline connected to the gate using peripheral decoder and control circuitry for programming (writing), reading, erasing, or other functions.
In most such array configurations the active regions of the individual flash cells are electrically isolated from one another by an isolation structure comprising an insulative material. This isolation structure may be formed similar to that of conventional shallow trench isolation (STI) fabrication methods before the formation of the ONO layers and the polysilicon gate layer.
As device densities increase and product dimensions decrease, it is desirable to reduce the size of the various structures and features associated with individual memory cells, sometimes referred to as scaling. However, the fabrication techniques used to produce conventional NAND flash memory arrays limit or inhibit the designer's ability to reduce array dimensions. For example, in a conventional manufacturing process whereby a conductive source line structure having a local interconnect may be formed in a NAND flash memory device, the STI is initially formed and filled, and an ONO layer is formed overlying the STI and a substrate. An anisotropic etch is then used to remove the STI and ONO in an array VSS region (ARVSS), leaving a narrow opening to the underlying polysilicon of the device. After an N+ implant to join MDD regions of the select source gate transistors to the ARVSS source line, a conductive silicide layer is formed in the silicon of the wafer, and other typical lithographic processes continue as usual.
However, because it is difficult for the anisotropic etch to reach the bottom of the STI through the narrow and critically aligned opening, the formation of the conductive silicide is difficult on lateral sidewalls and the bottom of the trench, and detrimentally may become disconnected therein. Thus, in order to scale the memory cell devices to facilitate increased device densities, it is desirable to provide the widest possible conductive array VSS structures while simplifying fabrication processes. However, current processes used to form such source line structures are difficult to produce in a reliable manner, and effectively limit the ability to scale the device to the desired performance specifications. Thus, there is a need for improved manufacturing techniques by which NAND flash memory devices may be scaled without sacrificing device yield and performance.